The present invention relates to elements which, in combination, clamp workpieces in a centrally orienting fashion for purposes of enabling working them. The basic field of employment includes lathes, but the utilization within the field of stationary working is preferred. Devices, elements and combinations thereof are known in the following variety and exhibit the following drawbacks.
The centering vise uses racks and pinions or left-handed or right-handed spindle threads in order to obtain centering movements for clamping. The rack and pinion arrangement is disadvantaged by the fact that holding forces are always just as high as the clamping force. There is thus little safety provided to counteract the effect of percussions and impacts as they inevitably occur during working. In the case of delicate workpieces which must not be clamped too tightly, this is indeed a significant drawback. Using opposing thread pitches is troublesome because of inaccuracies arising from the increase in thread play. This approach, moveover, is limited to manual operation. Both kinds of approaches are practiced with two-bracket arrangements only. The brackets, moreover, are either stationarily positioned or slidable on centering slides. This, however, restricts the range of clamping. Also it is a considerable disadvantages that tilting moments may have to be taken up by the movable brackets, a factor which immediately increases the friction in the guide structure. This effect drastically increases with the height of the brackets and requires long and expensive guide structures coupled with a limiting in the employment of low clamping heights.
Another device is the key hook chuck which quite frequently is realized by a piston shaped key way hook standing in the center and driving lying centering clamping slide. This approach is disadvantaged by the fact that the play of the standing piston is directly responsible for causing a play of the entire packing unit. Also, there is a limitation as to height owing to the tilting moments as described earlier. Moreover, in the case of the four brackets wedge hook packing, both directions of movement are interconnected so that only good fitting and matching workpieces can actually be centered. This, however, contradicts the very purpose of centering, namely, the elimination-through-centering of tolerances. Still other drawbacks, being particularly noticeable for stationary employment, are large vertical dimensions on account of the standing piston and its guiding structure. Also at best add-on brackets can be displaced on the centering slides. Hence the size variability as well as the mounting space requirement is always as large as the packing housing.
A third approach is the so-called shift lever chuck. It is, however, not very common. A vertical hook pin drives lying centering slides through angularly bent deflection levers. Again, just as in the case of the rack-and-pinion device, the holding back force is just as the clamping force. All the other drawbacks are the same as mentioned above, namely, limits on account of tilting moments, undue vertical dimensions limits concerning dimensional variability coupling of the directions of movement, etc.
A fourth approach is the spiral or scroll chuck. Herein radially positioned centering clamping slides are usually driven by Archimede's spirals. Limitations to this approach again include limits on account of tilting moments, coupling of the directions of movement; the dimensional variability is even more limited on account of the specific spiral; and specific to this approach are the problems posed by inaccuracies and play generally. Hence this kind of equipment has been limited in its use to manually and rather simple kinds of clamping.